Only when you put it in 4wd? That's kind of odd. Just out of curiosity, did you put manual hubs on? The reason I ask, is if you haven't, your axles are spinning all of the time anyway so I can't see why 4wd would change anything. Still kind weird.
Regardless, it sounds like your axle seal needs to be replaced. You need to remove your: Brake caliper, hub/rotor, caliper adapter, spindle then the front shaft. Then just remove the seal on the end of the axle tube, it's quite easy to do.
Obviously, while you're there, replace all seals involved and your u-joint if it ain't new.
One more thing, the parts stores often have a hard time finding that seal #, so go to our handy dandy part number index to find it.